# ifndef CPPAD_EXAMPLE_ABS_NORMAL_SIMPLEX_METHOD_HPP
# define CPPAD_EXAMPLE_ABS_NORMAL_SIMPLEX_METHOD_HPP
/* --------------------------------------------------------------------------
CppAD: C++ Algorithmic Differentiation: Copyright (C) 2003-17 Bradley M. Bell

CppAD is distributed under multiple licenses. This distribution is under
the terms of the
                    Eclipse Public License Version 1.0.

A copy of this license is included in the COPYING file of this distribution.
Please visit http://www.coin-or.org/CppAD/ for information on other licenses.
-------------------------------------------------------------------------- */
/*
$begin simplex_method$$
$spell
	hpp
	maxitr
	xout
$$

$section abs_normal: Solve a Linear Program Using Simplex Method$$

$head Syntax$$
$icode%ok% = simplex_method(%level%, %b%, %A%, %c%, %maxitr%, %xout%)
%$$

$head Prototype$$
$srcfile%example/abs_normal/simplex_method.hpp%
	0%// BEGIN PROTOTYPE%// END PROTOTYPE%
1%$$

$head Source$$
This following is a link to the source code for this example:
$cref/simplex_method.hpp/simplex_method.hpp/$$.

$head Problem$$
We are given
$latex A \in \B{R}^{m \times n}$$,
$latex b \in \B{R}^m$$,
$latex c \in \B{R}^n$$.
This routine solves the problem
$latex \[
\begin{array}{rl}
\R{minimize} &
g^T x \; \R{w.r.t} \; x \in \B{R}_+^n
\\
\R{subject \; to} & A x + b \leq 0
\end{array}
\] $$

$head Vector$$
The type $icode Vector$$ is a
simple vector with elements of type $code double$$.

$head level$$
This value is less than or equal two.
If $icode%level% == 0%$$,
no tracing is printed.
If $icode%level% >= 1%$$,
a trace $latex x$$ and the corresponding objective $latex z$$
is printed at each iteration.
If $icode%level% == 2%$$,
a trace of the simplex Tableau is printed at each iteration.

$head A$$
This is a $cref/row-major/glossary/Row-major Representation/$$ representation
of the matrix $latex A$$ in the problem.

$head b$$
This is the vector $latex b$$ in the problem.

$head c$$
This is the vector $latex c$$ in the problem.

$head maxitr$$
This is the maximum number of simplex iterations to try before giving up
on convergence.

$head xout$$
This argument has size is $icode n$$ and
the input value of its elements does no matter.
Upon return it is the primal variables corresponding to the problem solution.

$head ok$$
If the return value $icode ok$$ is true, a solution has been found.

$children%example/abs_normal/simplex_method.cpp
	%example/abs_normal/simplex_method.omh
%$$
$head Example$$
The file $cref simplex_method.cpp$$ contains an example and test of
$code simplex_method$$.
It returns true if the test passes and false otherwise.

$end
-----------------------------------------------------------------------------
*/
# include <cmath>
# include <cppad/utility/error_handler.hpp>
# include "abs_print_mat.hpp"

// BEGIN C++
namespace CppAD { // BEGIN_CPPAD_NAMESPACE

// BEGIN PROTOTYPE
template <class Vector>
bool simplex_method(
	size_t        level   ,
	const Vector& A       ,
	const Vector& b       ,
	const Vector& c       ,
	size_t        maxitr  ,
	Vector&       xout    )
// END PROTOTYPE
{	// number of equations
	size_t ne  = b.size();
	// number of x variables
	size_t nx = c.size();
	CPPAD_ASSERT_UNKNOWN( size_t(A.size()) == ne * nx );
	CPPAD_ASSERT_UNKNOWN( level <= 2 );
	//
	if( level > 0 )
	{	std::cout << "start simplex_method\n";
		CppAD::abs_print_mat("A", ne, nx, A);
		CppAD::abs_print_mat("b", ne,  1, b);
		CppAD::abs_print_mat("c", nx, 1, c);
	}
	//
	// variables (columns) in the Tableau:
	// x: the original primary variables with size n
	// s: slack variables, one for each equation
	// a: auxillary variables, one for each negative right hand size
	// r: right hand size for equations
	//
	// Determine number of auxillary variables
	size_t na = 0;
	for(size_t i = 0; i < ne; i++)
	{	if( b[i] > 0.0 )
			++na;
	}
	// size of columns in the Tableau
	size_t nc = nx + ne + na + 1;

	// number of rows in Tableau, the equations plust two objectives
	size_t nr = ne + 2;

	// Initilize Tableau as zero
	Vector T(nr * nc);
	for(size_t i = 0; i < nr * nc; i++)
		T[i] = 0.0;

	// initialize basic variable flag as false
	CppAD::vector<size_t> basic(nc);
	for(size_t j = 0; j < nc; j++)
		basic[j] = false;

	// For i = 0 , ... , m-1, place the Equations
	// sum_j A_{i,j} * x_j + b_i <= 0 in Tableau
	na = 0; // use as index of next auxillary variable
	for(size_t i = 0; i < ne; i++)
	{	if( b[i] > 0.0)
		{	// convert to - sum_j A_{i,j} x_j - b_i >= 0
			for(size_t j = 0; j < nx; j++)
				T[i * nc + j] = - A[i * nx + j];
			// slack variable has negative coefficient
			T[i * nc + (nx + i)] = -1.0;
			// auxillary variable is basic for this constraint
			T[i * nc + (nx + ne + na)] = 1.0;
			basic[nx + ne + na]        = true;
			// right hand side
			T[i * nc + (nc - 1)] = b[i];
			//
			++na;
		}
		else
		{	// sum_j A_{i,j} x_j + b_i <= 0
			for(size_t j = 0; j < nx; j++)
				T[i * nc + j] = A[i * nx + j];
			//  slack variable is also basic
			T[ i * nc + (nx + i) ]  = 1.0;
			basic[nx + i]           = true;
			// right hand side for equations
			T[ i * nc + (nc - 1) ] = - b[i];
		}
	}
	// na is back to its original value
	CPPAD_ASSERT_UNKNOWN( nc == nx + ne + na + 1 );
	//
	// place the equation objective equation in Tablueau
	// row ne corresponds to the equation z - sum_j c_j x_j = 0
	// column index for z is nx + ne + na
	for(size_t j = 0; j < nx; j++)
		T[ne * nc + j] = - c[j];
	//
	// row ne+1 corresponds to the equation w - a_0 - ... - a_{na-1} = 0
	// column index for w is nx + ne + na +1
	for(size_t j = 0; j < na; j++)
		T[(ne + 1) * nc + (nx + ne + j)] = -1.0;
	//
	// fix auxillary objective so coefficients in w
	// for auxillary variables are zero
	for(size_t k = 0; k < na; k++)
	{	size_t ja  = nx + ne + k;
		size_t ia  = ne;
		for(size_t i = 0; i < ne; i++)
		{	if( T[i * nc + ja] != 0.0 )
			{	CPPAD_ASSERT_UNKNOWN( T[i * nc + ja] == 1.0 );
				CPPAD_ASSERT_UNKNOWN( T[(ne + 1) * nc + ja] == -1.0 )
				CPPAD_ASSERT_UNKNOWN( ia == ne );
				ia = i;
			}
		}
		CPPAD_ASSERT_UNKNOWN( ia < ne );
		for(size_t j = 0; j < nc; j++)
			T[(ne + 1) * nc + j] += T[ia * nc + j];
		// The result in column ja is zero, avoid roundoff
		T[(ne + 1) * nc + ja] = 0.0;
	}
	//
	// index of current objective
	size_t iobj = ne;  // original objective z
	if( na > 0 )
		iobj = ne + 1; // auxillary objective w
	//
	// simplex interations
	for(size_t itr = 0; itr < maxitr; itr++)
	{	// current value for xout
		for(size_t j = 0; j < nx; j++)
		{	xout[j] = 0.0;
			if( basic[j] )
			{	// determine which row of column j is non-zero
				xout[j] = std::numeric_limits<double>::quiet_NaN();
				for(size_t i = 0; i < ne; i++)
				{	double T_ij = T[i * nc + j];
					CPPAD_ASSERT_UNKNOWN( T_ij == 0.0 || T_ij == 1.0 );
					if( T_ij == 1.0 )
					{	// corresponding value in right hand side
						xout[j] = T[ i * nc + (nc-1) ];
					}
				}
			}
		}
		if( level > 1 )
			CppAD::abs_print_mat("T", nr, nc, T);
		if( level > 0 )
		{	CppAD::abs_print_mat("x", nx, 1, xout);
			std::cout << "itr = " << itr;
			if( iobj > ne )
				std::cout << ", auxillary objective w = ";
			else
				std::cout << ", objective z = ";
			std::cout << T[iobj * nc + (nc - 1)] << "\n";
		}
		//
		// number of variables depends on objective
		size_t nv = nx + ne;   // (x, s)
		if( iobj == ne + 1 )
		{	// check if we have solved the auxillary problem
			bool done = true;
			for(size_t k = 0; k < na; k++)
				if( basic[nx + ne + k] )
					done = false;
			if( done )
			{	// switch to optimizing the original objective
				iobj = ne;
			}
			else
				nv = nx + ne + na; // (x, s, a)
		}
		//
		// determine variable with maximuim coefficient in objective row
		double cmax = 0.0;
		size_t jmax = nv;
		for(size_t j = 0; j < nv; j++)
		{	if( T[iobj * nc + j] > cmax )
			{	CPPAD_ASSERT_UNKNOWN( ! basic[j] );
				cmax = T[ iobj * nc + j];
				jmax = j;
			}
		}
		// check for solution
		if( jmax == nv )
		{	if( iobj == ne )
			{	if( level > 0 )
					std::cout << "end simplex_method\n";
				return true;
			}
			if( level > 0 )
				std::cout << "end_simples_method: no feasible solution\n";
			return false;
		}
		//
		// We will increase the j-th variable.
		// Determine which row will be the pivot row.
		double rmin = std::numeric_limits<double>::infinity();
		size_t imin = ne;
		for(size_t i = 0; i < ne; i++)
		{	if( T[i * nc + jmax] > 0.0 )
			{	double r =	T[i * nc + (nc-1) ] / T[i * nc + jmax];
				if( r < rmin )
				{	rmin = r;
					imin = i;
				}
			}
		}
		if( imin == ne )
		{	// not auxillary objective
			CPPAD_ASSERT_UNKNOWN( iobj == ne );
			if( level > 0 ) std::cout
				<< "end simplex_method: objective is unbounded below\n";
			return false;
		}
		double pivot = T[imin * nc + jmax];
		//
		// Which variable is changing from basic to non-basic.
		// Initilaize as not yet determined.
		size_t basic2not = nc;
		//
		// Divide row imin by pivot element
		for(size_t j = 0; j < nc; j++)
		{	if( basic[j] && T[imin * nc + j] == 1.0 )
			{	CPPAD_ASSERT_UNKNOWN( basic2not == nc );
				basic2not = j;
			}
			T[imin * nc + j] /= pivot;
		}
		// The result in column jmax is one, avoid roundoff
		T[imin * nc + jmax ] = 1.0;
		//
		// Check that we found the variable going from basic to non-basic
		CPPAD_ASSERT_UNKNOWN( basic2not < nv && basic2not != jmax );
		//
		// convert variable for column jmax to basic
		// and for column basic2not to non-basic
		for(size_t i = 0; i < nr; i++) if( i != imin )
		{	double r =	T[i * nc + jmax ] / T[imin * nc + jmax];
			// row_i = row_i - r * row_imin
			for(size_t j = 0; j < nc; j++)
				T[i * nc + j] -= r * T[imin * nc + j];
			// The result in column jmax is zero, avoid roundoff
			T[i * nc + jmax] = 0.0;
		}
		// update flag for basic variables
		basic[ basic2not ] = false;
		basic[ jmax ]      = true;
	}
	if( level > 0 ) std::cout
		<< "end simplex_method: maximum # iterations without solution\n";
	return false;
}
} // END_CPPAD_NAMESPACE
// END C++

# endif
